1. Field of the Invention
The present invention relates to a method of forming a coating of a polymer-based paint composition on a substrate.
The invention relates particularly, although by no means exclusively, to a method of forming a thin ornamental and/or protective coating of a polymer-based paint composition on a moving substrate that is in the form of a metal strip.
2. Description of the Related Art
The term xe2x80x9cthinxe2x80x9d as used herein is understood to mean a coating thickness of up to 60 micron.
Typically, the polymer-based paint composition includes a resin, a pigment and a cross-linking agent as the main constituents.
Typically, the invention is applicable to the production of pre-painted steel strip that is used as the starting material in the production of building cladding sheets and other steel products for the building industry, appliance cabinets, vehicle bodies, and many other sheet metal products.
It is known to form a coating of a polymer-based paint composition on a steel strip by methods which are characterised by using paint compositions that are solid at ambient temperature and are in the form of solid blocks as the source of the coating. The methods include progressively melting the blocks and applying the resultant liquid paint composition to a surface of the strip. In this context, the term xe2x80x9cliquidxe2x80x9d is understood herein to include (a) high viscosity liquids, that may approach soft, plastic solids in nature, and (b) easily flowing liquids.
U.S. Pat. No. 5,407,697 of Buecher et al discloses one such method which includes the steps of:
(a) pre-heating a metal strip to a pre-heat temperature that is above the glass transition temperature of a thermosetting polymer-based paint composition to be coated onto a surface of the strip;
(b) moving the pre-heated strip at a pre-determined strip speed;
(c) driving a solid block of the paint composition along an axis of the block into contact with the surface of the strip to be coated to cause the paint composition to be melted from the block and applied to the surface as a liquid deposit and thereafter carried away from the block with the moving strip;
(d) forming the liquid deposit into a liquid coating of the paint composition on the surface of the strip; and
(e) heating the liquid coating deposit to produce a solid coating of the paint composition covering at least part of the surface of the strip.
An object of the present invention is to provide an alternative method of forming a coating of a polymer-based paint composition on a steel strip.
In accordance with the present invention pellets rather than solid blocks of the polymer-based paint composition are used as the source of the coating on the metal strip or other suitable substrate.
The use of pellets rather than solid blocks: avoids the need to tailor block dimensions to be compatible to strip width dimensions; makes it possible to achieve more uniform application of paint across the strip; and enables transportation of larger amounts of paint than is common with solids blocks. In summary, the use of pellets rather than solid blocks enables more flexible materials handling both in transit and at the paint line.
More specifically, the present invention provides a method of forming a coating of a polymer-based paint composition on a substrate which includes the steps of:
(a) pre-heating the substrate to a temperature that is above the glass transition temperature of the polymer of the paint composition;
(b) supplying pellets of the paint composition to the surface of the substrate whereby the pellets progressively melt into a liquid state;
(c) forming a coating of the liquid paint composition on the substrate surface; and
(d) converting the liquid coating into a solid coating of the paint composition.
The term xe2x80x9cpelletsxe2x80x9d is understood herein to mean solids that have a particle size which ranges from a lower limit of 100 micron, as measured in a major dimension of the particles, to an upper limit of 5 cms, again as measured in a major dimension of the particles.
The pellets may be regular or irregular shapes.
The term xe2x80x9cpelletsxe2x80x9d is a term that is used in the thermoplastic industry to describe solids of relatively uniform shape and size distribution that are manufactured by cutting strands of extruded material. The term xe2x80x9cpelletsxe2x80x9d as used herein includes, but is not restricted, to this definition.
The term xe2x80x9cpelletsxe2x80x9d is also understood herein to comprehend solids that are otherwise known as xe2x80x9cchipsxe2x80x9d in the powder coating industry and which are of random shape and varying size distribution. The term xe2x80x9cpelletsxe2x80x9d as used herein includes, but is not restricted to this definition, and xe2x80x9cgranulesxe2x80x9d in the thermoplastic industry.
The term xe2x80x9cpelletsxe2x80x9d is also understood herein to exclude solids described as xe2x80x9cpowdersxe2x80x9d in the powder coating disadvantages of slow colour change-over, difficult spray booth clean-up, poor transfer efficiency onto metal strip and difficulties forming thin film coatings that are associated with spray coating technology.
The term xe2x80x9cpelletsxe2x80x9d is also understood to exclude xe2x80x9cblocksxe2x80x9d of the type described, by way of example, in U.S. Pat. No. 5,407,697. In this context, the use of pellets in the subject invention requires a less complicated delivery system to that of blocks and is an advantage in this respect.
The polymer-based paint composition is selected to have appropriate properties given that the paint composition will be in the form of pellets and in many instances the pellets will be used at a different location to that at which the pellets are manufactured and will need to be transported from the manufacturing site to the end use location. Typically, the paint composition should have a glass transition temperature above the temperature experienced by pellets during transportation to avoid substantial deformation/breakdown of the pellets.
Preferably the polymer-based paint composition is a thermoset paint composition.
In situations where the paint composition is a thermoset paint composition, preferably step (d) of converting the liquid coating into the solid coating of the paint composition includes heating the liquid coating to thermoset the paint composition.
Preferably step (b) of supplying pellets of the paint composition to the surface of the substrate includes supplying pellets while the substrate is moving at a controlled speed along a pre-determined path.
Preferably the method includes controlling the deposition rate of the paint composition on the substrate for a given substrate speed by controlling the mass feed rate of pellets to the substrate.
Preferably step (c) of forming the coating of the liquid paint composition includes:
(i) positioning a roller transverse to the moving substrate so that the roller applies pressure to the surface to be coated and thereby controls the thickness of the liquid coating on the substrate; and
(ii) controlling the deposition rate of the paint composition on the substrate so that a pool of liquid paint composition is established and maintained immediately upstream of the roller and is contained by the roller and the substrate.
The pool is a reservoir from which liquid paint composition is drawn by the substrate as it moves between the roller and the surface to be coated.
Step (b) of the method may include supplying pellets onto the moving substrate upstream of the pool or directly into the pool. In the latter case, and potentially in the former case, the pool will contain pellets that are in the process of melting into a liquid state.
The coating may be formed over the entire width of or part only of the width of the substrate.